Multi-state NROM device

ABSTRACT

An array of NROM flash memory cells configured to store at least two bits per four F 2 . Split vertical channels are generated along each side of adjacent pillars. A single control gate is formed over the pillars and in the trench between the pillars. The split channels can be connected by an n+ region at the bottom of the trench or the channel wrapping around the trench bottom. Each gate insulator is capable of storing a charge that is adequately separated from the other charge storage area due to the increased channel length.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.10/842,304, titled “MULTI-STATE NROM DEVICE,” filed May 10, 2004, nowU.S. Pat. No. 7,050,330 which is a continuation-in-part of U.S.application Ser. No. 10/738,408, filed Dec. 16, 2003 now U.S. Pat. No.7,220,634 that is assigned to the assignee of the present invention andincorporated herein by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to memory devices and inparticular the present invention relates to NROM flash memory deviceswith high storage density.

BACKGROUND OF THE INVENTION

Memory devices are typically provided as internal, semiconductor,integrated circuits in computers or other electronic devices. There aremany different types of memory including random-access memory (RAM),read only memory (ROM), dynamic random access memory (DRAM), synchronousdynamic random access memory (SDRAM), and flash memory. One type offlash memory is a nitride read only memory (NROM). NROM has some of thecharacteristics of flash memory but does not require the specialfabrication processes of flash memory. NROM integrated circuits can beimplemented using a standard CMOS process.

Flash memory devices have developed into a popular source ofnon-volatile memory for a wide range of electronic applications. Flashmemory devices typically use a one-transistor memory cell that allowsfor high memory densities, high reliability, and low power consumption.Common uses for flash memory include personal computers, personaldigital assistants (PDAs), digital cameras, and cellular telephones.Program code and system data such as a basic input/output system (BIOS)are typically stored in flash memory devices for use in personalcomputer systems.

As computers and software become more complex, greater amounts of memoryare required to store data. Memory capacity can be increased by reducingtransistor size (e.g., feature size “F”) and/or storing multiple bits inone cell. Performing both of these options simultaneously greatlyincreases memory capacity while increasing the speed and decreasing thepower requirements of the memory device. However, a problem withdecreased NROM flash memory size is that NROM flash memory celltechnologies have some scaling limitations. As dimensions are scaleddown it becomes difficult to maintain adequate separation betweenmultiple charge storage regions of the NROM cell.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora higher performance flash memory transistor that can store multiplebits per cell.

SUMMARY

The embodiments of the present invention encompass a nitride read onlymemory device comprising a substrate with a plurality of verticalpillars, each pillar having an upper doped region. A gate insulatorlayer is formed along facing sides of a first pillar and a second pillarof the plurality of vertical pillars. A control gate is formed overlyingthe gate insulator layers and the pillars. A lower doped region isformed under a trench located between the first and second pillars.During operation of the transistor, the lower doped region couples afirst channel that forms along the facing side of the first pillar and asecond channel that forms along the facing side of the second pillar. Inone embodiment, the lower doped region is not connected to an electricalcontact.

These and other embodiments, aspects, advantages, and features of thepresent invention will be set forth in part in the description whichfollows, and in part will become apparent to those skilled in the art byreference to the following description of the invention and referenceddrawings or by practice of the invention. The aspects, advantages, andfeatures of the invention are realized and attained by means of theinstrumentalities, procedures, and combinations particularly pointed outin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a semiconductor substrate portion atone stage in processing in accordance with an embodiment of the presentinvention.

FIG. 2 is a cross-sectional view of one embodiment of the substrateportion of FIG. 1 at a later stage in processing.

FIG. 3 is a cross-sectional view of one embodiment of the substrateportion of FIG. 2 at a later stage in processing.

FIG. 4 is a simplified plan view of a substrate portion showing aportion of a memory cell array, in accordance with an embodiment of thepresent invention.

FIG. 5 is a cross-sectional view illustrating a relationship between thestructures of FIGS. 1-3 and the plan view of FIG. 4, in accordance withan embodiment of the present invention.

FIG. 6 is a simplified plan view of a memory cell array illustrating aninterconnection arrangement for the memory cell array of FIG. 4, inaccordance with an embodiment of the present invention.

FIG. 7 is a cross-sectional view, taken along section lines 7-7 of FIG.6, illustrating part of an interconnection arrangement in accordancewith an embodiment of the present invention.

FIG. 8 is a cross-sectional view, taken along section lines 8-8 of FIG.6, illustrating part of an interconnection arrangement in accordancewith an embodiment of the present invention.

FIG. 9A is a block diagram of a metal oxide semiconductor field effecttransistor (MOSFET) in a substrate according to the teachings of theprior art.

FIG. 9B illustrates the MOSFET of FIG. 9A operated in the forwarddirection showing some degree of device degradation due to electronsbeing trapped in the gate oxide near the drain region over gradual use.

FIG. 9C is a graph showing the square root of the current signal (Ids)taken at the drain region of the conventional MOSFET versus the voltagepotential (VGS) established between the gate and the source region.

FIG. 10A is a diagram of a programmed MOSFET which can be used as amulti-state cell in accordance with an embodiment of the presentinvention.

FIG. 10B is a diagram suitable for explaining the method by which theMOSFET of the multi-state cell of the present invention can beprogrammed to achieve the embodiments of the present invention.

FIG. 10C is a graph plotting the current signal (Ids) detected at thedrain region versus a voltage potential, or drain voltage, (VDS) set upbetween the drain region and the source region (Ids vs. VDS) inaccordance with an embodiment of the present invention.

FIG. 11 illustrates a vertical nitride read only memory cell that ispart of a memory array of the present invention.

FIG. 12 illustrates an electrical equivalent circuit for the portion ofthe memory array shown in FIG. 11.

FIG. 13 is another electrical equivalent circuit useful in illustratinga read operation on the novel multi-state cell in accordance with anembodiment of the present invention.

FIG. 14 illustrates a portion of a memory array in accordance with anembodiment of the present invention.

FIG. 15A illustrates one embodiment of the gate insulator for theembodiments of the present invention having a number of layers.

FIG. 15B illustrates the conduction behavior of the multi-state cell ofthe embodiments of the present invention.

FIG. 16A illustrates the operation and programming of the multi-statecell in the reverse direction.

FIG. 16B illustrates the now programmed multi-state cell's operation inthe forward direction and differential read occurring in thisdifferential cell embodiment, e.g., 2 transistors in each cell.

FIG. 17 illustrates a cross-sectional view of one embodiment of an NROMsplit channel flash memory cell of the embodiments of the presentinvention.

FIG. 18 illustrates a cross-sectional view of another embodiment of anNROM flash memory cell of the embodiments of the present invention.

FIG. 19 illustrates an electrical schematic equivalent of theembodiments of FIGS. 17 and 18.

FIG. 20 illustrates one embodiment for a memory device in accordancewith an embodiment of the present invention.

FIG. 21 is a block diagram of one embodiment of an electronic system, orprocessor-based system, utilizing a multi-state cell constructed inaccordance with the embodiments of the present invention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings that form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention.

The terms wafer and substrate used in the following description includeany structure having an exposed surface with which to form theintegrated circuit (IC) structure of the invention. The term substrateis understood to include semiconductor wafers. The term substrate isalso used to refer to semiconductor structures during processing, andmay include other layers that have been fabricated thereupon. Both waferand substrate include doped and undoped semiconductors, epitaxialsemiconductor layers supported by a base semiconductor or insulator, aswell as other semiconductor structures well known to one skilled in theart. The term conductor is understood to include semiconductors, and theterm insulator is defined to include any material that is lesselectrically conductive than the materials referred to as conductors.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims, along with the full scope of equivalents towhich such claims are entitled.

FIG. 1 is a cross-sectional view of a semiconductor substrate portion 20at one stage in processing, in accordance with an embodiment of thepresent invention. The portion 20 includes etched or incised recesses22, doped regions 24 and 26 and caps 28. The etched recesses 22 formtrenches extending along an axis into and out of the page of FIG. 1.

In one embodiment, the doped regions 24 are implanted n+ regions. In oneembodiment, the doped regions 24 are formed by a blanket implant. In oneembodiment, the caps 28 are dielectric caps and may be formed usingconventional silicon nitride and conventional patterning techniques. Inone embodiment, the etched recesses 22 are then etched usingconventional plasma etching techniques. In one embodiment, the dopedregions 26 are then doped by implantation to form n+ regions. The etchedor incised recesses 22 may be formed by plasma etching, laser-assistedtechniques or any other method presently known or that may be developed.In one embodiment, the recesses 22 are formed to have substantiallyvertical sidewalls relative to a top surface of the substrate portion20. In one embodiment, substantially vertical means at 90 degrees to thesubstrate surface, plus or minus ten degrees.

FIG. 2 provides a cross-sectional view of the substrate portion 20 ofFIG. 1 at a later stage in processing, in accordance with an embodimentof the present invention. The portion 20 of FIG. 2 includes thick oxideregions 32, ONO regions 34 formed on sidewalls 36 of the recesses 22,gate material 38 and a conductive layer 40. In one embodiment, the gatematerial 38 comprises conductively-doped polycrystalline silicon.

In one embodiment, conventional techniques are employed to oxidize thedoped regions 24 and 26 preferentially with respect to sidewalls 36. Asa result, the thick oxide regions 32 are formed at the same time as athinner oxide 42 on the sidewalls 36. These oxides also serve to isolatethe doped regions 24 and 26 from what will become transistor channelsalong the sidewalls 36. Other techniques for isolation may be employed.For example, in one embodiment, high density plasma grown oxides may beemployed. In one embodiment, spacers may be employed.

In one embodiment, conventional techniques are then employed to providea nitride layer 44 and an oxide layer 46, as is described, for example,in “NROM: A Novel Localized Trapping, 2-Bit Nonvolatile Memory Cell”, byBoaz Eitan et al., IEEE Electron Device Letters, Vol. 21, No. 11,November 2000, pp. 543-545, IEEE Catalogue No. 0741-3106/00, or in “ATrue Single-Transistor Oxide-Nitride-Oxide EEPROM Device” by T. Y. Chanet al., IEEE Electron Device Letters, Vol. EDL-8, No. 3, March, 1987,pp. 93-95, IEEE Catalogue No. 0741-3106/87/0300-0093.

In one embodiment, the thin oxide 42, nitride layer 44 and oxide layer46 combine to form the ONO layer 34, such as is employed in SONOSdevices, while the polysilicon 38 forms a control gate. In operation,application of suitable electrical biases to the doped regions 24, 26and the control gate 38 cause hot majority charge carriers to beinjected into the nitride layer 44 and become trapped, providing athreshold voltage shift and thus providing multiple, alternative,measurable electrical states representing stored data. “Hot” chargecarriers are not in thermal equilibrium with their environment. In otherwords, hot charge carriers represent a situation where a population ofhigh kinetic energy charge carriers exist. Hot charge carriers may beelectrons or holes.

SONOS devices are capable of storing more than one bit per gate 38.Typically, the hot carriers are injected into one side 47 or 47′ of theONO layer 34, adjacent a contact, such as the region 24 or the region26, that provides a high electrical field.

By reversing the polarity of the potentials applied to the regions 24and 26, charge may be injected into the other side 47′ or 47 of the ONOlayer 34. Thus, four electronically-discriminable and distinct statescan be easily provided with a single gate 38. As a result, the structureshown in FIG. 2 is capable of storing at least four bits per gate 38.

FIG. 3 is a cross-sectional view of the substrate portion 20 of FIG. 1at an alternative stage in processing, in accordance with an embodimentof the present invention. The embodiment shown in FIG. 3 includes theoxide regions 32 and 42, but a floating gate 48 is formed on the thinoxide region 42. A conventional oxide or nitride insulator 49 is formedon the floating gate 48, followed by deposition of gate material 38.Floating gate devices are known and operate by injecting hot chargecarriers, which may comprise electrons or holes, into the floating gate48.

Floating gate devices can be programmed to different charge levels thatcan be electrically distinct and distinguishable. As a result, it ispossible to program more data than one bit into each floating gatedevice, and each externally addressable gate 38 thus corresponds to morethan one stored bit. Typically, charge levels of 0, Q, 2Q and 3Q mightbe employed, where Q represents some amount of charge corresponding to areliably distinguishable output signal.

FIG. 4 is a simplified plan view of a substrate portion showing aportion of a memory cell array 50, in accordance with an embodiment ofthe present invention. FIG. 4 also provides examples of pitch P, widthW, space S and minimum feature size F, as described in the Background.An exemplary memory cell area 52, the physical area of a singletransistor, can be seen to be about one F². Wordlines 54 are formed fromthe conductive layer 40, and bitlines 56 and 58 are formed.

FIG. 5 is a simplified side view, in section, illustrating arelationship between the structures of FIGS. 1-3 and the plan view ofFIG. 4, in accordance with an embodiment of the present invention. Thetrenches 22 correspond to bitlines 56 and 58, as is explained below inmore detail with reference to FIGS. 6-8.

The density of memory arrays such as that described with reference toFIGS. 1-5 can require interconnection arrangements that differ fromprior art memory arrays. One embodiment of a new type of interconnectionarrangement useful with such memory systems is described below withreference to FIGS. 6-8.

FIG. 6 is a simplified plan view illustrating an interconnectionarrangement 60 for the memory cell array 50 of FIG. 4, in accordancewith an embodiment of the present invention. The interconnectionarrangement 60 includes multiple patterned conductive layers 62 and 64,separated by conventional interlevel dielectric material 65 (FIGS. 7 and8). The views in FIG. 6-8 have been simplified to show correspondencewith the other Figures and to avoid undue complexity. Shallow trenchisolation regions 67 isolate selected portions from one another.

FIG. 7 is a cross-sectional view, taken along section lines 7-7 of FIG.6, illustrating part of an interconnection arrangement in accordancewith an embodiment of the present invention.

FIG. 8 is a cross-sectional view, taken along section lines 8-8 of FIG.6, illustrating part of an interconnection arrangement in accordancewith an embodiment of the present invention.

With reference to FIGS. 6-8, the patterned conductive layer 62 extendsupward to nodes 70, 70′, 70″ and establishes electrical communicationbetween the conductive layers 62 and selected portions of the dopedregion 24. The patterned conductive layer 62 stops at the line denoted72, 72′.

Similarly, other portions of the patterned conductive layer 62 extendfrom the line denoted 74, 74′ and extend upward, providing electricalcommunication from nodes 76, 76′, 76″ to other circuit elements. Thenodes 76, 76′, 76″ provide contact to selected portions of the dopedregion 24.

In contrast, patterned conductive layers 64 extend from top to bottom ofFIG. 6 and electrically couple to nodes 78, 78″ and thus to doped region26.

Such is but on example of a simplified interconnection arrangementsuitable for use with the memory devices of FIGS. 1-5. Otherarrangements are possible.

FIG. 9A is useful in illustrating the conventional operation of a MOSFETsuch as can be used in a DRAM array. FIG. 9A illustrates the normal hotelectron injection and degradation of devices operated in the forwarddirection. As is explained below, since the electrons are trapped nearthe drain they are not very effective in changing the devicecharacteristics.

FIG. 9A is a block diagram of a metal oxide semiconductor field effecttransistor (MOSFET) 101 in a substrate 100. The MOSFET 101 includes asource region 102, a drain region 104, a channel region 106 in thesubstrate 100 between the source region 102 and the drain region 104. Agate 108 is separated from the channel region 108 by a gate oxide 110. Asourceline 112 is coupled to the source region 102. A bitline 114 iscoupled to the drain region 104. A wordline 116 is coupled to the gate108.

In conventional operation, a drain to source voltage potential (Vds) isset up between the drain region 104 and the source region 102. A voltagepotential is then applied to the gate 108 via a wordline 116. Once thevoltage potential applied to the gate 108 surpasses the characteristicvoltage threshold (Vt) of the MOSFET a channel 106 forms in thesubstrate 100 between the drain region 104 and the source region 102.Formation of the channel 106 permits conduction between the drain region104 and the source region 102, and a current signal (Ids) can bedetected at the drain region 104.

In operation of the conventional MOSFET of FIG. 9A, some degree ofdevice degradation does gradually occur for MOSFETs operated in theforward direction by electrons 117 becoming trapped in the gate oxide110 near the drain region 104. This effect is illustrated in FIG. 9B.However, since the electrons 117 are trapped near the drain region 104they are not very effective in changing the MOSFET characteristics.

FIG. 9C illustrates this point. FIG. 9C is a graph showing the squareroot of the current signal (Ids) taken at the drain region versus thevoltage potential (VGS) established between the gate 108 and the sourceregion 102. The change in the slope of the plot of √{square root over(Ids)} versus VGS represents the change in the charge carrier mobilityin the channel 106.

In FIG. 9C, ΔVT represents the minimal change in the MOSFET's thresholdvoltage resulting from electrons gradually being trapped in the gateoxide 110 near the drain region 104, under normal operation, due todevice degradation. This results in a fixed trapped charge in the gateoxide 110 near the drain region 104. Slope 1 represents the chargecarrier mobility in the channel 106 for FIG. 9A having no electronstrapped in the gate oxide 110. Slope 2 represents the charge mobility inthe channel 106 for the conventional MOSFET of FIG. 9B having electrons117 trapped in the gate oxide 110 near the drain region 104. As shown bya comparison of slope 1 and slope 2 in FIG. 9C, the electrons 117trapped in the gate oxide 110 near the drain region 104 of theconventional MOSFET do not significantly change the charge mobility inthe channel 106.

There are two components to the effects of stress and hot electroninjection. One component includes a threshold voltage shift due to thetrapped electrons and a second component includes mobility degradationdue to additional scattering of carrier electrons caused by this trappedcharge and additional surface states. When a conventional MOSFETdegrades, or is “stressed,” over operation in the forward direction,electrons do gradually get injected and become trapped in the gate oxidenear the drain. In this portion of the conventional MOSFET there isvirtually no channel underneath the gate oxide. Thus the trapped chargemodulates the threshold voltage and charge mobility only slightly.

Applicant has previously described programmable memory devices andfunctions based on the reverse stressing of MOSFET's in a conventionalCMOS process and technology in order to form programmable address decodeand correction. (See generally, L. Forbes, W. P. Noble and E. H. Cloud,“MOSFET technology for programmable address decode and correction,” U.S.patent application Ser. No. 09/383,804). That disclosure, however, didnot describe multi-state memory cell solutions, but rather addressdecode and correction issues.

According to the teachings of the present invention, normal MOSFETs,including split-channel NROM devices, can be programmed by operation inthe reverse direction and utilizing avalanche hot electron injection totrap electrons in the gate oxide of the MOSFET. When the programmedMOSFET is subsequently operated in the forward direction the electronstrapped in the oxide are near the source and cause the channel to havetwo different threshold voltage regions. The novel programmed MOSFETs ofthe present invention conduct significantly less current thanconventional MOSFETs, particularly at low drain voltages. Theseelectrons will remain trapped in the gate oxide unless negative gatevoltages are applied. The electrons will not be removed from the gateoxide when positive or zero gate voltages are applied. Erasure can beaccomplished by applying negative gate voltages and/or increasing thetemperature with negative gate bias applied to cause the trappedelectrons to be re-emitted back into the silicon channel of the MOSFET.(See generally, L. Forbes, E. Sun, R. Alders and J. Moll, “Field inducedre-emission of electrons trapped in SiO₂,” IEEE Trans. Electron Device,vol. ED-26, no. 11, pp. 1816-1818 (November 1979); S. S. B. Or, N.Hwang, and L. Forbes, “Tunneling and Thermal emission from adistribution of deep traps in SiO₂,” IEEE Trans. on Electron Devices,vol. 40, no. 6, pp. 1100-1103 (June 1993); S. A. Abbas and R. C.Dockerty, “N-channel IGFET design limitations due to hot electrontrapping,” IEEE Int. Electron Devices Mtg., Washington D.C., December1975, pp. 35-38).

FIGS. 10A-10C are useful in illustrating the present invention in whicha much larger change in device characteristics is obtained byprogramming the device in the reverse direction and subsequently readingthe device by operating it in the forward direction.

FIG. 10A is a diagram of a programmed MOSFET that can be used as amulti-state cell according to the teachings of the present invention. Asshown in FIG. 10A the multi-state cell 201 includes a MOSFET in asubstrate 200 which has a first source/drain region 202, a secondsource/drain region 204, and a channel region 206 between the first andsecond source/drain regions, 202 and 204. In one embodiment, the firstsource/drain region 202 includes a source region 202 for the MOSFET andthe second source/drain region 204 includes a drain region 204 for theMOSFET. FIG. 10A further illustrates a gate 208 separated from thechannel region 206 by a gate oxide 210. A first transmission line 212 iscoupled to the first source/drain region 202 and a second transmissionline 214 is coupled to the second source/drain region 204. In oneembodiment, the first transmission line includes a sourceline 212 andthe second transmission line includes a bit line 214.

As stated above, multi-state cell 201 is comprised of a programmedMOSFET. This programmed MOSFET has a charge 217 trapped in the gateoxide 210 adjacent to the first source/drain region 202 such that thechannel region 206 has a first voltage threshold region (Vt1) and asecond voltage threshold region (Vt2) in the channel 206. In oneembodiment, the charge 217 trapped in the gate oxide 210 adjacent to thefirst source/drain region 202 includes a trapped electron charge 217.According to the teachings of the present invention and as described inmore detail below, the multi-state cell can be programmed to have one ofa number of charge levels trapped in the gate insulator adjacent to thefirst source/drain region 202 such that the channel region 206 will havea first voltage threshold region (Vt1) and a second voltage thresholdregion (Vt2) and such that the programmed multi-state cell operates atreduced drain source current.

FIG. 10A illustrates the Vt2 in the channel 206 is adjacent the firstsource/drain region 202 and that the Vt1 in the channel 206 is adjacentthe second source/drain region 204. According to the teachings of thepresent invention, Vt2 has a higher voltage threshold than Vt1 due tothe charge 217 trapped in the gate oxide 217 adjacent to the firstsource/drain region 202. Multiple bits can be stored on the multi-statecell 201.

FIG. 10B is a diagram suitable for explaining the method by which theMOSFET of the multi-state cell 201 of the present invention can beprogrammed to achieve the embodiments of the present invention. As shownin FIG. 10B the method includes programming the MOSFET in a reversedirection. Programming the MOSFET in the reverse direction includesapplying a first voltage potential V1 to a drain region 204 of theMOSFET. In one embodiment, applying a first voltage potential V1 to thedrain region 204 of the MOSFET includes grounding the drain region 204of the MOSFET as shown in FIG. 10B. A second voltage potential V2 isapplied to a source region 202 of the MOSFET. In one embodiment,applying a second voltage potential V2 to the source region 202 includesapplying a high positive voltage potential (VDD) to the source region202 of the MOSFET, as shown in FIG. 10B. A gate potential VGS is appliedto a gate 208 of the MOSFET. In one embodiment, the gate potential VGSincludes a voltage potential which is less than the second voltagepotential V2, but which is sufficient to establish conduction in thechannel 206 of the MOSFET between the drain region 204 and the sourceregion 202. As shown in FIG. 10B, applying the first, second and gatepotentials (V1, V2, and VGS respectively) to the MOSFET creates a hotelectron injection into a gate oxide 210 of the MOSFET adjacent to thesource region 202. In other words, applying the first, second and gatepotentials (V1, V2, and VGS respectively) provides enough energy to thecharge carriers, e.g. electrons, being conducted across the channel 206that, once the charge carriers are near the source region 202, a numberof the charge carriers get excited into the gate oxide 210 adjacent tothe source region 202. Here the charge carriers become trapped.

In one embodiment of the present invention, the method is continued bysubsequently operating the MOSFET in the forward direction in itsprogrammed state during a read operation. Accordingly, the readoperation includes grounding the source region 202 and precharging thedrain region a fractional voltage of VDD. If the device is addressed bya wordline coupled to the gate, then its conductivity will be determinedby the presence or absence of stored charge in the gate insulator. Thatis, a gate potential can be applied to the gate 208 by a wordline 216 inan effort to form a conduction channel between the source and the drainregions as done with addressing and reading conventional DRAM cells.

However, now in its programmed state, the conduction channel 206 of theMOSFET will have a first voltage threshold region (Vt1) adjacent to thedrain region 204 and a second voltage threshold region (Vt2) adjacent tothe source region 202, as explained and described in detail inconnection with FIG. 10A. According to the teachings of the presentinvention, the Vt2 has a greater voltage threshold than the Vt1 due tothe hot electron injection 217 into a gate oxide 210 of the MOSFETadjacent to the source region 202.

FIG. 10C is a graph plotting a current signal (Ids) detected at thesecond source/drain region 204 versus a voltage potential, or drainvoltage, (VDS) set up between the second source/drain region 204 and thefirst source/drain region 202 (Ids vs. VDS). In one embodiment, VDSrepresents the voltage potential set up between the drain region 204 andthe source region 202. In FIG. 10C, the curve plotted as D1 representsthe conduction behavior of a conventional MOSFET which is not programmedaccording to the teachings of the present invention. The curve D2represents the conduction behavior of the programmed MOSFET, describedabove in connection with FIG. 10A, according to the teachings of thepresent invention. As shown in FIG. 10C, for a particular drain voltage,VDS, the current signal (IDS2) detected at the second source/drainregion 204 for the programmed MOSFET (curve D2) is significantly lowerthan the current signal (IDS1) detected at the second source/drainregion 204 for the conventional MOSFET which is not programmed accordingto the teachings of the present invention. Again, this is attributed tothe fact that the channel 206 in the programmed MOSFET of the presentinvention has two voltage threshold regions and that the voltagethreshold, Vt2, near the first source/drain region 202 has a highervoltage threshold than Vt1 near the second source/drain region due tothe charge 217 trapped in the gate oxide 217 adjacent to the firstsource/drain region 202.

Some of these effects have recently been described for use in adifferent device structure, called an NROM, for flash memories. Thislatter work in Israel and Germany is based on employing charge trappingin a silicon nitride layer in a non-conventional flash memory devicestructure. (See generally, B. Eitan et al., “Characterization of ChannelHot Electron Injection by the Subthreshold Slope of NROM device,” IEEEElectron Device Lett., Vol. 22, No. 11, pp. 556-558, (November 2001); B.Etian et al., “NROM: A novel localized Trapping, 2-Bit NonvolatileMemory Cell,” IEEE Electron Device Lett., Vol. 21, No. 11, pp. 543-545,(November 2000)). Charge trapping in silicon nitride gate insulators wasthe basic mechanism used in MNOS memory devices (see generally, S. Sze,Physics of Semiconductor Devices, Wiley, N.Y., 1981, pp. 504-506),charge trapping in aluminum oxide gates was the mechanism used in MIOSmemory devices (see generally, S. Sze, Physics of Semiconductor Devices,Wiley, N.Y., 1981, pp. 504-506), and Applicant has previously disclosedcharge trapping at isolated point defects in gate insulators (seegenerally, L. Forbes and J. Geusic, “Memory using insulator traps,” U.S.Pat. No. 6,140,181, issued Oct. 31, 2000).

In contrast to the above work, the present invention disclosesprogramming a MOSFET in a reverse direction to trap one of a number ofcharge levels near the source region and reading the device in a forwarddirection to form a multi-state memory cell based on a modification ofDRAM technology.

Prior art DRAM technology generally employs silicon oxide as the gateinsulator. Further the emphasis in conventional DRAM devices is placedon trying to minimize charge trapping in the silicon oxide gateinsulator. According to the teachings of the present invention, avariety of insulators are used to trap electrons more efficiently thanin silicon oxide. That is, in the present invention, the multi-statememory cell employs charge trapping in gate insulators such as, wetsilicon oxide, silicon nitride, silicon oxynitride SON, silicon richoxide SRO, aluminum oxide Al₂O₃, composite layers of these insulatorssuch as oxide and then silicon nitride, or oxide and then aluminumoxide, or multiple layers as oxide-nitride-oxide. While the chargetrapping efficiency of silicon oxide may be low such is not the case forsilicon nitride or composite layers of silicon oxide and nitride.

FIG. 11 illustrates a vertical NROM memory cell that is part of a memoryarray according to the teachings of the present invention. The memory inFIG. 11 is shown illustrating a number of vertical pillars, ormulti-state cells, 301-1 and 301-2 formed according to the teachings ofthe present invention. As one of ordinary skill in the art willappreciate upon reading this disclosure, the number of vertical pillarsare formed in rows and columns extending outwardly from a substrate 303.

As shown in FIG. 11, the number of vertical pillars, 301-1 and 301-2 areseparated by a number of trenches 340. According to the teachings of thepresent invention, the number of vertical pillars, 301-1 and 301-2,serve as transistors including a first source/drain region, 302-1 and302-2, respectively. The first source/drain region, 302-1 and 302-2, iscoupled to a sourceline 304. As shown in FIG. 11, the sourceline 304 isformed in a bottom of the trenches 340 between rows of the verticalpillars, 301-1 and 301-2. In one embodiment, according to the teachingsof the present invention, the sourceline 304 is formed from a dopedregion implanted in the bottom of the trench. A second source/drainregion, 306-1 and 306-2 respectively, is coupled to a bitline (notshown). A channel region 305 is located between the first and the secondsource/drain regions.

As shown in FIG. 11, a gate 309 is separated from the channel region 305by a gate insulator 307 in the trenches 340 along rows of the verticalpillars, 301-1 and 301-2. In one embodiment, according to the teachingsof the present invention, the gate insulator 307 includes a gateinsulator 307 selected from the group of silicon dioxide (SiO₂) formedby wet oxidation, silicon oxynitride (SON), silicon rich oxide (SRO),and aluminum oxide (Al₂O₃). In another embodiment, according to theteachings of the present invention, the gate insulator 307 includes agate insulator 307 selected from the group of silicon rich aluminumoxide insulators, silicon rich oxides with inclusions of nanoparticlesof silicon, silicon oxide insulators with inclusions of nanoparticles ofsilicon carbide, and silicon oxycarbide insulators. In anotherembodiment, according to the teachings of the present invention, thegate insulator 307 includes a composite layer 307. In this embodiment,the composite layer 307 includes a composite layer 307 selected from thegroup of an oxide-aluminum oxide (Al₂O₃)-oxide composite layer, andoxide-silicon oxycarbide-oxide composite layer. In another embodiment,the composite layer 307 includes a composite layer 307, or anon-stoichiometric single layer, of two or more materials selected fromthe group of silicon (Si), titanium (Ti), and tantalum (Ta). In anotherembodiment, according to the teachings of the present invention, thegate insulator 307 includes an oxide-nitride-oxide (ONO) gate insulator307.

FIG. 12 illustrates an electrical equivalent circuit 400 for the portionof the memory array shown in FIG. 11. As shown in FIG. 12, a number ofvertical multi-state cells, 401-1 and 401-2, are provided. Each verticalmulti-state cell, 401-1 and 401-2, includes a first source/drain region,402-1 and 402-2, a second source/drain region 406-1 and 406-2, a channelregion 405 between the first and the second source/drain regions, and agate 409 separated from the channel region by a gate insulator 407.

FIG. 12 further illustrates a number of bit lines, 411-1 and 411-2,coupled to the second source/drain region 406-1 and 406-2 of eachmulti-state cell. In one embodiment, as shown in FIG. 12, the number ofbit lines, 411-1 and 411-2, are coupled to the second source/drainregion 406-1 and 406-2 along rows of the memory array. A number of wordlines, such as wordline 413 in FIG. 12, are coupled to the gate 409 ofeach multi-state cell along columns of the memory array. A number ofsourcelines, such as common sourceline 415, are coupled to the firstsource/drain regions, e.g. 402-1 and 402-2, along columns of thevertical multi-state cells, 401-1 and 401-2, such that adjacent pillarscontaining these transistors share the common sourceline 415.

In one embodiment, column adjacent pillars include a transistor whichoperates as a vertical multi-state cell, e.g. 401-1, on one side of ashared trench, the shared trench separating rows of the pillars asdescribed in connection with FIG. 11, and a transistor which operates asa reference cell, e.g. 401-2, having a programmed conductivity state onthe opposite side of the shared trench. In this manner, according to theteachings of the present invention and as described in more detailbelow, at least one of multi-state cells can be programmed to have oneof a number of charge levels trapped in the gate insulator, showngenerally as 417, adjacent to the first source/drain region, e.g. 402-1,such that the channel region 405 will have a first voltage thresholdregion (Vt1) and a second voltage threshold region (Vt2) and such thatthe programmed multi-state cell operates at reduced drain sourcecurrent.

FIG. 13 is another electrical equivalent circuit useful in illustratinga read operation on the novel multi-state cell 500 according to theteachings of the present invention. The electrical equivalent circuit inFIG. 13 represents a programmed vertical multi-state cell. As explainedin detail in connection with FIG. 11, the programmed verticalmulti-state cell 500 includes a vertical metal oxide semiconductor fieldeffect transistor (MOSFET) 500 extending outwardly from a substrate. TheMOSFET has a source region 502, a drain region 506, a channel region 505between the source region 502 and the drain region 506, and a gate 509separated from the channel region 505 by a gate insulator, showngenerally as 507.

As shown in FIG. 13 a wordline 513 is coupled to the gate 509. Asourceline 504, formed in a trench adjacent to the vertical MOSFET asdescribed in connection with FIG. 11, is coupled to the source region502. A bit line, or data line 511 is coupled to the drain region 506.The multi-state cell 500 shown in FIG. 13 is an example of a programmedmulti-state cell 500 having one of a number of charge levels trapped inthe gate insulator, shown generally as 517, adjacent to the firstsource/drain region, 502, such that the channel region 505 will have afirst voltage threshold region (Vt1) and a second voltage thresholdregion (Vt2) and such that the programmed multi-state cell 500 operatesat reduced drain source current. According to the teachings of thepresent invention, the second voltage threshold region (Vt2) is now ahigh voltage threshold region that is greater than the first voltagethreshold region (Vt1).

FIG. 14 illustrates a portion of a memory array 600 according to theteachings of the present invention. The memory in FIG. 14 is shownillustrating a pair of multi-state cells 601-1 and 601-2 formedaccording to the teachings of the present invention. As one of ordinaryskill in the art will understand upon reading this disclosure, anynumber of multi-state cells can be organized in an array, but for easeof illustration only two are displayed in FIG. 14.

As shown in FIG. 14, a first source/drain region, 602-1 and 602-2respectively, is coupled to a sourceline 604. A second source/drainregion, 606-1 and 606-2 respectively, is coupled to a bitline, 608-1 and608-2 respectively. Each of the bitlines, 608-1 and 608-2, couple to asense amplifier, shown generally at 610. A wordline, 612-1 and 612-2respectively, is couple to a gate, 614-1 and 614-2 respectively, foreach of the multi-state cells, 601-1 and 601-2. According to theteachings of the present invention, the wordlines, 612-1 and 612-2, runacross or are perpendicular to the rows of the memory array 600.

Finally, a write data/precharge circuit is shown at 624 for coupling afirst or a second potential to bitline 608-1. As one of ordinary skillin the art will understand upon reading this disclosure, the writedata/precharge circuit 624 is adapted to couple either a ground to thebitline 608-1 during a write operation in the reverse direction, oralternatively to precharge the bitline 608-1 to fractional voltage ofV_(DD) during a read operation in the forward direction. As one ofordinary skill in the art will understand upon reading this disclosure,the sourceline 604 can be biased to a voltage higher than V_(DD) duringa write operation in the reverse direction, or alternatively groundedduring a read operation in the forward direction.

As shown in FIG. 14, the array structure 600, including multi-statecells 601-1 and 601-2, has no capacitors. Instead, according to theteachings of the present invention, the first source/drain region orsource region, 602-1 and 602-2, are coupled directly to the sourceline604. In order to write, the sourceline 604 is biased to voltage higherthan VDD and the devices stressed in the reverse direction by groundingthe data or bit line, 608-1 or 608-2. If the multi-state cell, 601-1 or601-2, is selected by a word line address, 612-1 or 612-2, then themulti-state cell, 601-1 or 601-2, will conduct and be stressed withaccompanying hot electron injection into the cells gate insulatoradjacent to the source region, 602-1 or 602-2. As one of ordinary skillin the art will understand upon reading this disclosure, a number ofdifferent charge levels can be programmed into the gate insulatoradjacent to source region such that the cells is used as a differentialcell and/or the cell is compared to a reference or dummy cell, as shownin FIG. 14, and multiple bits can be stored on the multi-state cell.

During read the multi-state cell, 601-1 or 601-2, is operated in theforward direction with the sourceline 604 grounded and the bit line,608-1 or 608-2, and respective second source/drain region or drainregion, 606-1 and 606-2, of the cells precharged to some fractionalvoltage of VDD. If the device is addressed by the word line, 612-1 or612-2, then its conductivity will be determined by the presence orabsence of the amount of stored charge trapped in the gate insulator asmeasured or compared to the reference or dummy cell and so detectedusing the sense amplifier 610. The operation of DRAM sense amplifiers isdescribed, for example, in U.S. Pat. Nos. 5,627,785; 5,280,205; and5,042,011, all assigned to Micron Technology Inc., and incorporated byreference herein. The array would thus be addressed and read in theconventional manner used in DRAM's, but programmed as multi-state cellsin a novel fashion.

In operation the devices would be subjected to hot electron stress inthe reverse direction by biasing the sourceline 604, and read whilegrounding the sourceline 604 to compare a stressed multi-state cell,e.g. cell 601-1, to an unstressed dummy device/cell, e.g. 601-2, asshown in FIG. 14. The write and possible erase feature could be usedduring manufacture and test to initially program all cells or devices tohave similar or matching conductivity before use in the field. Likewise,the transistors in the reference or dummy cells, e.g. 601-2, can allinitially be programmed to have the same conductivity states. Accordingto the teachings of the present invention, the sense amplifier 610 canthen detect small differences in cell or device characteristics due tostress induced changes in device characteristics during the writeoperation.

As one of ordinary skill in the art will understand upon reading thisdisclosure such arrays of multi-state cells are conveniently realized bya modification of DRAM technology. According to the teachings of thepresent invention a gate insulator of the multi-state cell includes gateinsulators selected from the group of thicker layers of SiO₂ formed bywet oxidation, SON silicon oxynitride, SRO silicon rich oxide, Al₂O₃aluminum oxide, composite layers and implanted oxides with traps (L.Forbes and J. Geusic, “Memory using insulator traps,” U.S. Pat. No.6,140,181, issued Oct. 31, 2000). Conventional transistors for addressdecode and sense amplifiers can be fabricated after this step withnormal thin gate insulators of silicon oxide.

FIGS. 15A-15B and 16A-16B are useful in illustrating the use of chargestorage in the gate insulator to modulate the conductivity of themulti-state cell according to the teachings of the present invention.That is, FIGS. 15A-16B illustrate the operation of the novel multi-statecell 701 formed according to the teachings of the present invention. Asshown in FIG. 15A, the gate insulator 707 has a number of layers, e.g.an ONO stack, where layer 707A is the oxide layer closest to the channel705 and a nitride layer 707B is formed thereon.

In the embodiment shown in FIG. 15A the oxide layer 707A is illustratedhaving a thickness of approximately 6.7 nm or 67 Å (roughly 10⁻⁶ cm). Inthe embodiment shown in FIG. 15A a multi-state cell is illustratedhaving dimensions of 0.1 μm (10⁻⁵ cm) by 0.1 μm. For purposes ofillustration, the charge storage region near the source can reasonablyhave dimensions of 0.1 micron (1000 Å) by 0.02 micron (200 Å) in a 0.1micron technology. If the gate oxide 707A nearest the channel 705 is 67Å then a charge of 100 electrons will cause a threshold voltage shift inthis region of 1.6 Volts since the oxide capacitance is about 0.5micro-Farad (μF) per square centimeter. If the transistor has a totaleffective oxide thickness of 200 Å then a change in the thresholdvoltage of only 0.16 Volts near the source, corresponding to 10electrons, is estimated to change the transistor current by 4 microAmperes (μA). The sense amplifier described in connection with FIG. 14,which is similar to a DRAM sense amplifier, can easily sense this chargedifference on the data or bitlines. In this embodiment, the sensedcharge difference on the data or bitlines will be 40 femto Coulombs (fC)over a sense period of 10 nano seconds (nS).

To illustrate these numbers, the capacitance, Ci, of the structuredepends on the dielectric constant, ∈i, (which for silicon dioxide SiO₂equates to 1.06/3×10⁻¹² F/cm), and the thickness of the insulatinglayers, t, (given here as 6.7×10⁻⁷ cm), such that Ci=∈i/t=((1.06×10⁻¹²F/cm/(3×6.7×10⁻⁷ cm))=0.5×10⁻⁶ Farads/cm² (F/cm²). This value taken overthe charge storage region near the source, e.g. 20 nm×100 nm or 2×10⁻¹¹cm², results in a capacitance value of Ci=10⁻¹⁷ Farads. Thus, for achange in the threshold voltage of ΔV=1.6 Volts the stored charge mustbe Q=C×ΔV=(10⁻¹⁷ Farads×1.6 Volts)=1.6×10⁻¹⁷ Coulombs. Since Q=Nq, thenumber of electrons stored is approximately Q/q=(1.6×10⁻¹⁷Coulombs/1.6×10⁻¹⁹ Coulombs) or 100 electrons.

In effect, the programmed multi-state cell, or modified MOSFET is aprogrammed MOSFET having a charge trapped in the gate insulator adjacentto a first source/drain region, or source region, such that the channelregion has a first voltage threshold region (Vt1) and a second voltagethreshold region (Vt2), where Vt2 is greater than Vt1, and Vt2 isadjacent the source region such that the programmed MOSFET operates atreduced drain source current. For Δ Q=100 electrons in the dimensionsgiven above, if the transistor has a total effective oxide thickness of200 Å then a change in the threshold voltage of only 0.16 Volts near thesource, corresponding to 10 electrons, is estimated to change thetransistor current by 4 micro Amperes (μA). As stated above, the senseamplifier described in connection with FIG. 14, which is similar to aDRAM sense amplifier, can easily sense this charge difference on thedata or bitlines. The sensed charge difference on the data or bitlineswill be 40 femto Coulombs (fC) over a sense period of 10 nano seconds(nS) for this representative one of a number of stored charge levelsaccording to the teachings of the present invention. A number ofdifferent charge levels can be programmed into the gate insulatoradjacent to source region such that the cell is used as a differentialcell and/or the cell is compared to a reference or dummy cell, as shownin FIG. 14, and multiple bits can be stored on the multi-state cell ofthe present invention.

FIG. 15B aids to further illustrate the conduction behavior of the novelmulti-state cell of the present invention. The electrical equivalentcircuit shown in FIG. 15B illustrates a multi-state cell 701 having anequivalent oxide thickness of 200 Å. The charge storage region near thesource 702 can reasonably have a length dimension of 0.02 micron (20 nm)in a 0.1 micron technology with a width dimension of 0.1 micron (100nm). Therefore, for a change in the drain source voltage (Δ VDS) in thisregion an electric field of E=(0.1 V/2×10⁻⁶ cm)=0.5×105 V/cm or 5×104V/cm is provided. The drain current is calculated using the formulaID=μCox×(W/L)×(Vgs−Vt)×Δ VDS. In this example, μCox=μCi is taken as 50μA/V2 and W/L=5. Appropriate substitution into the drain currentprovides ID=(50 μA/V2×5×0.16 Volts×0.1 Volts)=2.5×1.6 μA=4 μA. As notedabove this drain current ID corresponds to 10 electrons trapped in thegate insulator, or charge storage region 707 near the source 702. Sensedover a period of 10 nanoseconds (ns) produces a current on the bitlineof 40 fC (e.g. 4 μA×10 nS=40×10⁻¹⁵ Coulombs).

FIGS. 16A and 16B illustrate the operation and programming of the novelmulti-state cell as described above. However, FIGS. 16A and 16B alsohelp illustrate an alternative array configuration where adjacentdevices are compared and one of the devices on the opposite side of ashared trench is used as a dummy cell transistor or reference device.Again, the reference devices can all be programmed to have the sameinitial conductivity state.

FIG. 16A illustrates the operation and programming of the novelmulti-state cell in the reverse direction. A transistor 801-1 on oneside of the trench (as described in connection with FIG. 11) is stressedby grounding its respective drain line, e.g. 811-1. The drain line 811-2for the transistor 801-2 on the opposite side of the trench is leftfloating. A voltage is applied to the shared sourceline 804 located atthe bottom of the trench (as described in connection with FIG. 11) thatnow acts as a drain. The neighboring (shared trench)/column adjacenttransistors, 801-1 and 801-2, share a gate 807 and the wordline 813,e.g. polysilicon gate lines, coupling thereto run across or areperpendicular to the rows containing the bit and source lines, e.g.811-1, 811-2, and 804. A gate voltage is applied to the gates 807. Herethe multi-state cell 801-1 will conduct and be stressed withaccompanying hot electron injection into the cells gate insulator 817adjacent to the source region 802-1.

FIG. 16B illustrates the now programmed multi-state cell's operation inthe forward direction and differential read occurring in a thisdifferential cell embodiment, e.g. 2 transistors in each cell. To readthis state the drain and source (or ground) have the normal connectionsand the conductivity of the multi-state cell is determined. The drainlines 811-1 and 811-2 have the normal forward direction potentialapplied. The shared sourceline 804 located at the bottom of the trench(as described in connection with FIG. 11) is grounded and once againacts as a source. And, a gate voltage is applied to the gates 807.

As one of ordinary skill in the art will understand upon reading thisdisclosure, a number of different charge levels can be programmed intothe gate insulator 817 adjacent to source region 802-1 and compared tothe reference or dummy cell, 802-2. Thus, according to the teachings ofpresent invention multiple bits can be stored on the multi-state cell.

As stated above, these novel multi-state cells can be used in aDRAM-like array. Two transistors can occupy an area of 4F² (F=theminimum lithographic feature size) when viewed from above, or eachmemory cell consisting of one transistor utilizing an area of 2F². Eachtransistor can now, however, store many bits so the data storage densityis much higher than one bit for each 1F² unit area. Using a reference ordummy cell for each memory transistor where the reference transistor isin close proximity, e.g. the embodiment shown in FIGS. 16A and 16B vs.that shown in FIG. 12, results in better matching characteristics oftransistors, but a lower memory density.

FIG. 17 illustrates a cross-sectional view of an embodiment of avertical NROM flash memory transistor of the present invention. Use ofvertical device structure increases the channel length while keeping thearea occupied by the cell to four square feature sizes (i.e., 4F²).

This embodiment is comprised of one control gate 1704 and two splitchannels 1710 and 1711 along the sides of two pillars 1701 and 1702respectively. An n+ region 1703 under the trench connects the twochannel segments 1710 and 1711 during transistor operation so that thestructure acts like two transistors in series. In the presentembodiment, the two transistors in series have at least two chargestorage areas. Alternate embodiments may include different quantities ofstorage areas.

The transistors each have a nitride storage region 1706 and 1708 that,in one embodiment, is part of an ONO gate insulator layer. Charge can bestored in the gate insulator in either or both channel segments 1710 and1711. The n+ regions 1720 and 1721 in the upper portions of adjacentpillars 1701 and 1702 act as either a source region or a drain region,depending on the direction of operation of the transistors. Thesource/drain regions are coupled by data/bit lines that extend along thez-axis, substantially perpendicular to the wordline/control gate 1704.

The bottom of the trench and the tops of the pillars have an oxidedielectric material between the substrate and the control gate 1704.Alternate embodiments may use other types of dielectric materials.

The embodiment of FIG. 17 illustrates n+ regions being doped into ap-type the substrate. However, alternate embodiments may dope p+ regionsinto an n-type substrate.

FIG. 18 illustrates a cross-sectional view of another embodiment of avertical NROM flash memory transistor of the present invention. Chargecan be stored at either end of the channel 1801. As in the embodiment ofFIG. 17, the n+ regions 1803 and 1804 act as source/drain regions andtheir function depends on the direction of operation of the transistor.

FIG. 19 illustrates a schematic diagram of the electrical equivalent ofthe embodiments of FIGS. 17 and 18. The transistors illustrated in FIGS.17 and 18 are shown as two field effect transistors (FETs) operating inseries with the drain of one coupled to the source of the other.

The reference numbers of FIG. 17 have been used in FIG. 19 to illustratethe relation of the components of FIG. 19 to those of FIG. 17. WhileFIG. 19 illustrates the drain 1721 and source 1720 being a certainorientation, if the transistor is operated in the opposite direction,the drain and source regions are opposite.

The floating n+ diffusion area 1703 couples the separate parts 1710 and1711 of the channel. There is no electrical contact on the n+ region1703. The single gate 1704 couples the two transistors.

The flash memory cells of the embodiments of FIGS. 17 and 18 can befabricated using modifications to the fabrication techniques discussedpreviously. The structures of FIGS. 17 and 18 use the same etchedvertical pillars but the NROM flash memory structure forms two channelsalong the sidewalls of the adjacent pillars and an n+ region forms atransistor channel along the bottom of the trench. The single controlgate is formed in the trenches between the pillars and the n+source/drain regions at the tops of the pillars form the data/bit lines.In the embodiment of FIG. 18, a gate insulator and the control gate forma part of the channel across the bottom of the trench.

Conventional channel hot electron injection can be used for programmingin which a source region is grounded and a drain region is biased with apositive voltage while the control gate has a positive programmingvoltage applied. Conventional negative gate Fowler-Nordheim tunnelingcan be used for erasing the cells. In the present embodiments, thedevice can be used for two bit storage. The charge is stored near thedrain and the device is read in the reverse direction. Either end of thechannel can be used as a drain in response to the direction of operationand a charge stored at both ends of the channel near the n+ regions atthe surface.

In alternate embodiments, substrate enhanced hot electron injection canbe used for programming the NROM cells of the present invention.Additionally, substrate enhanced band-to-band tunneling induced hot holeinjection can be used for erasing the cells.

The ONO layer is only one embodiment for a gate insulator of the NROMcells of the present invention. Additional gate insulator compositionsinclude: oxide-nitride-aluminum oxide composite layers, oxide-aluminumoxide-oxide composite insulators, oxide-silicon oxycarbide-oxidecomposite layers, as well as other composite layers. Additionally, thegate insulator may be thicker than normal silicon oxides formed by wetoxidation and not annealed, silicon rich oxides with inclusions ofnanoparticles of silicon, silicon oxynitride layer (not compositelayers), silicon rich aluminum oxide insulators (not composite layers),silicon oxycarbide insulators (not composite layers), silicon oxideinsulators with inclusions of nanoparticles of silicon carbide, as wellas other non-stoichiometric single layers of gate insulators of two ormore commonly used insulator materials including, but not limited to,Si, N, Al, Ti, Ta, Hf, and La.

In FIG. 20 a memory device is illustrated according to the teachings ofthe present invention. In one embodiment, the device is an NROM deviceof the present invention. In an alternate embodiment, it can be a DRAMdevice of the present invention.

The memory device 940 contains a memory array 942, row and columndecoders 944, 948 and a sense amplifier circuit 946. The memory array942 consists of a plurality of multi-state cells 900, formed accordingto the teachings of the present invention whose word lines 980 and bitlines 960 are commonly arranged into rows and columns, respectively. Thebit lines 960 of the memory array 942 are connected to the senseamplifier circuit 946, while its word lines 980 are connected to the rowdecoder 944. Address and control signals are input on address/controllines 961 into the memory device 940 and connected to the column decoder948, sense amplifier circuit 946 and row decoder 944 and are used togain read and write access, among other things, to the memory array 942.

The column decoder 948 is connected to the sense amplifier circuit 946via control and column select signals on column select lines 962. Thesense amplifier circuit 946 receives input data destined for the memoryarray 942 and outputs data read from the memory array 942 overinput/output (I/O) data lines 963. Data is read from the cells of thememory array 942 by activating a word line 980 (via the row decoder944), which couples all of the memory cells corresponding to that wordline to respective bit lines 960, which define the columns of the array.One or more bit lines 960 are also activated. When a particular wordline 980 and bit lines 960 are activated, the sense amplifier circuit946 connected to a bit line column detects and amplifies the conductionsensed through a given multi-state cell, where in the read operation thesource region of a given cell is couple to a grounded array plate (notshown), and transferred its bit line 960 by measuring the potentialdifference between the activated bit line 960 and a reference line whichmay be an inactive bit line. The operation of Memory device senseamplifiers is described, for example, in U.S. Pat. Nos. 5,627,785;5,280,205; and 5,042,011, all assigned to Micron Technology Inc., andincorporated by reference herein.

FIG. 21 is a block diagram of an electronic system, or processor-basedsystem, 1000 utilizing multi-state memory cells 1012 constructed inaccordance with the embodiments of the present invention. That is, themulti-state memory cells 1012 utilize the DRAM or NROM flash memorycells as described previously.

The processor-based system 1000 may be a computer system, a processcontrol system, or any other system employing a processor and associatedmemory. The system 1000 includes a central processing unit (CPU) 1002 orother controller circuit (e.g., a microprocessor) that communicates withthe multi-state memory 1012 and an I/O device 1008 over a bus 1020. Thebus 1020 may be a series of buses and bridges commonly used in aprocessor-based system, but for convenience purposes only, the bus 1020has been illustrated as a single bus. A second I/O device 1010 isillustrated, but is not necessary to practice the invention. Theprocessor-based system 1000 can also includes read-only memory (ROM)1014 and may include peripheral devices such as a floppy disk drive 1004and a compact disk (CD) ROM drive 1006 that also communicates with theCPU 1002 over the bus 1020 as is well known in the art.

It will be appreciated by those skilled in the art that additionalcircuitry and control signals can be provided, and that the memorydevice 1000 has been simplified to help focus on the invention. At leastone of the multi-state cells in NROM 1012 includes a programmed MOSFEThaving a charge trapped in the gate insulator adjacent to a firstsource/drain region, or source region, such that the channel region hasa first voltage threshold region (Vt1) and a second voltage thresholdregion (Vt2), where Vt2 is greater than Vt1, and Vt2 is adjacent thesource region such that the programmed MOSFET operates at reduced drainsource current.

It will be understood that the embodiment shown in FIG. 21 illustratesan embodiment for electronic system circuitry in which the novel memorycells of the present invention are used. The illustration of system1000, as shown in FIG. 21, is intended to provide a generalunderstanding of one application for the structure and circuitry of thepresent invention, and is not intended to serve as a completedescription of all the elements and features of an electronic systemusing the novel memory cell structures. Further, the invention isequally applicable to any size and type of memory device 1000 using thenovel memory cells of the present invention and is not intended to belimited to that described above. As one of ordinary skill in the artwill understand, such an electronic system can be fabricated insingle-package processing units, or even on a single semiconductor chip,in order to reduce the communication time between the processor and thememory device.

Applications containing the novel memory cell of the present inventionas described in this disclosure include electronic systems for use inmemory modules, device drivers, power modules, communication modems,processor modules, and application-specific modules, and may includemultilayer, multichip modules. Such circuitry can further be asubcomponent of a variety of electronic systems, such as a clock, atelevision, a cell phone, a personal computer, an automobile, anindustrial control system, an aircraft, and others.

CONCLUSION

The novel multi-state cells of the present invention can be used in anNROM flash memory array. Two transistors can occupy an area of 4F² whenviewed from above. Each such transistor can now, however, store multiplebits so that the data storage density is much higher than one bit foreach 1F² unit area.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. A method for manufacturing a split-channel transistor, the method comprising: incising a substrate to form a plurality of trenches, each pair of trenches defining a pillar; forming a diffusion region in an upper portion of each pillar; forming a lower diffusion region under each trench; forming a nitride storage region only on each facing side of adjacent pillars; forming an oxide dielectric on the pillars and in the trenches, under the nitride storage regions, such that an oxide dielectric, without a nitride layer, is formed on top of each pillar and on the bottom of each trench; and forming a single control gate overlying the pillars and in the plurality of trenches wherein during a programming operation of the transistor, a channel is adapted to form along the facing sides of adjacent pillars and are connected by the lower diffusion region, the lower diffusion region not being coupled to an electrical contact.
 2. The method of claim 1 and further including forming a dielectric material in each trench between the substrate and the control gate.
 3. The method of claim 1 wherein the control gate is polysilicon.
 4. The method of claim 1 wherein the split-channel transistor forms two field effect transistors connected in series.
 5. The method of claim 1 wherein a first diffusion region in a first pillar is a drain region and a second diffusion region in a second, adjacent pillar is a source region.
 6. The method of claim 1 wherein forming the diffusion regions comprise creating n+ regions in a p-type substrate.
 7. A method for fabricating a split-channel flash memory transistor, the method comprising: forming a trench that is defined by two adjacent pillars; forming an upper source/drain region in an upper portion of each pillar; forming a lower diffusion region under the trench; forming a nitride region only along vertical facing sides of the two pillars; forming an oxide dielectric on the pillars and in the trenches, under the nitride regions, such that an oxide dielectric, without a nitride layer, is formed on top of each pillar and on the bottom of each trench; and forming a single control gate overlying the two pillars and in the trench wherein a channel segment is formed along each vertical facing side of the two pillars that are connected by the lower diffusion region, the lower diffusion region not being connected to an electrical contact.
 8. The method of claim 7 wherein forming the upper source/drain region comprises doping an n+ region in a p-type substrate.
 9. The method of claim 7 wherein forming the lower diffusion region comprises doping an n+ region in a p-type substrate.
 10. The method of claim 7 wherein the nitride regions are adapted to store a charge during transistor operation such that the transistor has two charge storage regions.
 11. A method for fabricating a split-channel flash memory array, the method comprising: forming a plurality of trenches in a substrate, each pair of trenches defining a pillar; forming an upper source/drain region at the top of each pillar; forming a lower diffusion region under each trench; forming a nitride layer only along each vertical facing side of each pillar; forming an oxide layer on the pillars and in the trenches, under the nitride layers, such that an oxide layer, without a nitride layer, is formed on top of each pillar and on the bottom of each trench; and forming a single control gate overlying the pillars and in the plurality of trenches wherein a channel region exists along each vertical facing side of the pillars in which a channel forms during operation of a transistor, adjacent facing channel regions connected into a single channel region by the lower diffusion region, the lower diffusion region having no electrical contact.
 12. The method of claim 11 wherein the function of the upper source/drain regions is determined by biasing of each transistor of the memory array.
 13. The method of claim 11 wherein the substrate is a p-type substrate and the upper source/drain regions and lower diffusion region are n+ doped regions.
 14. The method of claim 11 wherein the nitride layer is adapted to act as a charge storage layer during operation of each transistor. 